1. Field of the Invention
The present invention is broadly concerned with improved chemical oxygen generating compositions, and more particularly is concerned with improved chemical oxygen generating compositions containing strontium peroxide as a chlorine suppressant, reaction rate modifier, catalyst and a secondary oxygen source. Although the application of the oxygen generating compositions in this invention can be used as an emergency source of breathable oxygen, such as in passenger aircraft, the application of the oxygen generating compositions of the invention is not limited to aviation. The formulation can also have other applications such as in submarines, diving and mountain climbing, where it is useful to furnish a convenient reliable supply of oxygen gas of breathable quality.
2. Description of Related Art
Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates and perchlorates have long been used as an emergency oxygen source, such as in passenger airplanes. A typical chemical oxygen generator for aviation contains a chemical oxygen generating core. The chemical core typically has several layers of chemical mixtures with different compositions and thus different reaction rates. Multiple layers are used in the core instead of a single composition to match the oxygen requirements determined by the descend profile of an airplane following a loss of cabin pressure. The chemical oxygen generating core typically has a cylindrical shape with a taper, with a recess at one end to hold an ignition pellet. The ignition pellet is typically ignited by a primer. The heat from the ignition pellet then initiates the decomposition of the chlorates and/or perchlorates in the body of the chemical oxygen generating core and starts oxygen generation. The chemical ingredients for each layer of the chemical core are mixed together and a small amount of water is added to the mix to facilitate the mixing. The wet mixes are then poured into a mold in order and pressed into the chemical oxygen generating cores. The water in the mixes serves as a binder and a lubricant to facilitate mixing and pressing. Without water as a lubricant it would be extremely difficult to press the dry chemical powders into chemical cores with desired shape and density. The water in the chemical cores is then removed through drying in an oven.
Such chemical oxygen generating compositions typically contain one or more of sodium chlorate, lithium perchlorate and potassium perchlorate as an oxygen source. The chlorate and perchlorates decompose to produce oxygen gas once the reaction is initiated. A catalyst is typically used to facilitate the decomposition and a metal powder is used as a fuel to supply some extra heat to sustain the decomposition reaction. During the decomposition of sodium chlorate, lithium perchlorate, and potassium perchlorate, a small amount of chlorine gas is formed through side reactions. Chlorine gas is toxic and needs to be removed, usually with a filter installed inside of the chemical oxygen generator. Since big and heavy filters are undesirable for aviation applications, an alkaline compound is typically mixed in the oxygen generating compositions to suppress the chlorine formation and to scrap it if formed.
A chemical oxygen generator for an airplane is commonly preprogrammed according to the descend profile of the airplane and must meet the minimum oxygen flow requirement at all times during a descent. The oxygen generation rate can be erratic and uneven. In order for the trough of the oxygen flow rate to still be above the minimum flow rate requirement, extra chemicals need to be used. This increases the weight and is undesirable. A reaction rate modifier needs to be used to have a relatively smooth oxygen flow rate from the oxygen generator. It is preferable if one chemical can serve as both the chlorine suppressant and reaction rate modifier. BaO2, Li2O2, Na2O2 and Na2O, Ca(OH)2, and MgO have been used in chemical oxygen generating cores as chlorine suppressants and reaction rate modifiers. However, there are problems associated with the use of these alkaline chemicals.
Barium peroxide is considered toxic by government agencies around the world. Disposal of expended oxygen generators and scraps containing barium peroxide is difficult and expensive. Many customers prefer to use oxygen generators that do not contain barium compounds.
Sodium oxide and peroxide are air sensitive. They react with moisture and CO2 in the air. Therefore, moisture and CO2 need to be avoided during the manufacturing of the chemical cores containing sodium oxide and peroxide. In the absence of water as a binder and lubricant, it is very difficult to press the dry chemical powder mixture to desired shape and density, and the process is much more difficult and expensive. In addition, sodium oxide and peroxide are very caustic and corrosive. They react with water to form sodium hydroxide, which is also caustic and corrosive, and releases a large amount of heat.
Lithium peroxide is also unstable and it reacts with water to form, lithium hydroxide, particularly at elevated temperatures. Lithium peroxide is a very strong inhibitor for the decomposition of sodium chlorate, potassium perchlorate and lithium perchlorate. Only a small amount, such as a fraction of one percent, of lithium peroxide can be used in the chemical oxygen generating compositions. It takes a prolonged mixing to distribute such a small amount of lithium peroxide uniformly in the chemical oxygen generating compositions. Lithium peroxide is also relatively expensive, and a reliable supplier that supplies large quantity of lithium peroxide does not exist in the US.
Calcium hydroxide is better than the alkaline oxide and peroxides described above but it also has some undesirable properties. Calcium hydroxide suppresses the side reaction for chlorine formation and it can modify the oxygen generating reaction to have a smooth oxygen generation rate. But it has minimal catalytic activity and an additional catalyst has to be used with it. Since it is a very strong inhibitor for the decomposition of the chlorate and perchlorates, only a small amount such as a fraction of one percent can be used. It takes a prolonged mixing to distribute such a small amount in the compositions. It decomposes at around 580° C. to calcium oxide and water. The water can increase the moisture level in the oxygen produced by the oxygen generators. It is necessary to use a prolonged drying for the chemical cores so that the moisture level in the oxygen does not exceed the maximum level allowed by some applications.
Magnesium oxide and hydroxide have also been used as chlorine suppressants and reaction rate modifiers. However, their inhibiting ability is not high enough and their alkalinity is not high enough, and a relatively high percentage of loading has to be used. Since magnesium oxide and hydroxide are light and fluffy, it is difficult to achieve high density for the chemical oxygen generating cores if a high loading of these two compounds are used. For aviation applications, the volume of a product is also critical. One needs to save as much space as possible for the payloads. In addition, magnesium oxide and hydroxide do not produce oxygen, and a high loading would reduce the oxygen yield, which is undesirable. Magnesium hydroxide decomposes at 350° C. to magnesium oxide and water, which can increase moisture level in the oxygen, and this can be a problem for some applications.
Because of the problems associated with the chemical oxygen generating compositions given in the prior art it is necessary to have improved chemical oxygen generating compositions that are environmentally friendly, inexpensive, stable in the presence of water, easy to be manufactured, and have high oxygen yield. These compositions can be mixed and pressed with water as a mixing and pressing aid. The chemical oxygen cores made from these compositions have smooth oxygen generation rates and produce reduced levels of chlorine in the oxygen generated.
It is thus desirable to provide an improved oxygen generating formulation containing an additive that works as a chlorine suppressant, catalyst, reaction rate modifier and an additional oxygen source, for generating a smooth flow of oxygen with low levels of chlorine contamination and low toxicity, and which is easier and less expensive to manufacture. Chemical oxygen generating compositions that use strontium peroxide as a chlorine suppressant, reaction rate modifier, catalyst and secondary oxygen source in the present invention meet these needs.